In some systems there is a need to provide a secondary power source, for example, a capacitor or battery, that is used to power the system after primary power is removed, in order to allow for graceful cleanup of any data processing and/or data storage. Solid state drives (SSDs) with mapping and cached data stored in dynamic random access memory (DRAM) is such a system. When primary power is removed, the controllers on the SSDs need some time to migrate any required data safely from DRAM to the non-volatile memory storage.
The existing method to allow the logical “OR”ing of power sources is two parallel diodes, typically Schottky diodes, with a common cathode providing power to the circuit and each anode connected to a respective power source. FIG. 1 illustrates an example of such an arrangement. A primary power source 102 providing a VSUP voltage is coupled to an output node VOUT through diode 110 and a secondary power source 104 providing a VBACKUP voltage is coupled to the VOUT node through diode 112. A load, represented by resistance 106, is coupled to the VOUT node. In operation, the primary power source 102 provides power to the VOUT node by forward biasing the diode 110. The voltage at the VOUT node as driven by the primary power source 102 is sufficient to prevent the diode 112 from being forward biased. As a result, the secondary power source 104 does not provide power to the VOUT node. In response to the primary power source 102 no longer providing power to the VOUT node (e.g., the primary power source 102 is disconnected), the voltage of the VOUT node will decrease and cause the diode 112 to be forward biased. As a result, the secondary power source 104 provides power to the VOUT node instead of the primary power source 102. If the primary power source 102 again provides power (e.g., the primary power source 102 is reconnected), the diode 110 becomes forward biased so that the VSUP voltage is provided to the VOUT node and the diode 112 is no longer forward biased so that the secondary power source 104 is no longer providing power to the VOUT node.
A drawback of the configuration illustrated in FIG. 1 is the diodes 110, 112 waste power at a rate of about (0.4 V×I), where I is the current supplied to the system load. For example, for a system that draws two amps from a 12 Volt supply, the immediate loss power is about 0.8 Watts from the diodes, or 3% of the total power. For a 5V supply, the immediate loss is 8%. In power limited systems, the inefficiency detracts from the maximum power available for the system to operate, and decreases the maximum performance the system can provide. In addition to the performance issue, the loss in the diode is dissipated as heat which must be further dissipated from the system.